Etuhu, a summer signing from Fulham, had been sidelined since Christmas with the problem and suffered a recurrence during his comeback for Rovers' reserve side against Sunderland on Monday.
Boss Michael Appleton confirmed the news after the goalless npower Championship draw with Leeds and the 30-year-old joins a mounting injury list.
Martin Olsson suffered a freak training ground accident on Friday when he was struck by a ball, causing internal bleeding in his eye.
The Sweden international full-back is facing a month on the sidelines alongside midfielder Lee Williamson, who suffered an ankle injury at Hull on Tuesday night, and forward Colin Kazim-Richards was added to the walking wounded after hobbling off early in the second half against Neil Warnock's side.
"There have been a lot of things that have gone against us over the last 48 hours," Appleton told Radio Rovers. "We've had three players go down before the game who are going to be out for four to six weeks.
"Now with Kaz going down early in the second half we'll wait for the diagnosis and have a better idea of what is going to happen.
"I think he landed on the boy and twisted his ankle a little bit. He felt something at the back of his ankle, probably something similar to the Lee Williamson one in a way. Hopefully it isn't as serious as that and we can get him back on the pitch as soon as possible.
"With Martin he is pretty much going to be out for four to six weeks, so that isn't fantastic news, it is a shame for the lad and a shame for the football club as he is a key part of what we want to do."